text: Prior to the widespread use of fluorescence in the past three decades radioactivity was the most common label.
  The primary advantage of fluorescence over radiotracers is that it does not require radiological controls and their associated expenses and safety measures.  The decay of radioisotopes may limit the shelf life of a reagent, requiring its replacement and thus increasing expenses.  Several fluorescent molecules can be used simultaneously (given that they do not overlap, cf. FRET), whereas with radioactivity two isotopes can be used (tritium and a low energy isotope, e.g. P due to different intensities) but require special equipment (a tritium screen and a regular phosphor-imaging screen, a specific dual channel detector, e.g.  [https://web.archive.org/web/20090105194225/http://www.biospacelab.com/html/microimager.html]).
  Fluorescence is not necessary easier or more convenient to use because fluorescence requires specialized equipment of its own and because quenching makes absolute and/or reproducible quantification difficult.
  The primary disadvantage of fluorescence versus radiotracers is a significant biological problem:  chemically tagging a molecule with a fluorescent dye radically changes the structure of the molecule, which in turn can radically change the way that molecule interacts with other molecules.  In contrast, intrinsic radiolabeling of a molecule can be done without altering its structure in any way.  For example, substituting a H-3 for a hydrogen atom or C-14 for a carbon atom does not change the conformation, structure, or any other property of the molecule, it's just switching forms of the same atom.  Thus an intrinsically radiolabeled molecule is identical to its unlabeled counterpart.
  Measurement of biological phenomena by radiotracers is always direct.  In contrast, many life science fluorescence applications are indirect, consisting of a fluorescent dye increasing, decreasing, or shifting in wavelength emission upon binding to the molecule of interest.